Cyclic or multi-pass Time of Flight (“ToF”) mass spectrometers are known. In a cyclic or multi-pass Time of Flight mass spectrometer ions undergo multiple passes along the same flight tube during a fixed time period before exiting the flight tube and being directed to an ion detector. This arrangement extends the total length of a flight path within a compact device and thus improves the maximum mass resolution.
However, a particular problem with cyclic or multi-pass Time of Flight separators is that the resulting mass spectrum comprises ion peaks that are not organised in mass to charge ratio order since faster ions are allowed to overtake slower ions within the device during the multiple passes.
In order to construct a meaningful mass spectrum, the number of passes that each species of ions has taken whilst inside the flight tube needs to be determined either by calculation or by estimation. This is a non-trivial problem.
US 2012/0112060 (Kinugawa) discloses a multi-turn Time of Flight mass spectrometer.
U.S. Pat. No. 8,258,467 (Kajihara) discloses a mass analysing method and mass spectrometer.
U.S. Pat. No. 8,164,054 (Nishiguchi) discloses a mass analysis method and mass analysis system.
US 2011/0231109 (Furuhashi) discloses a mass analysis data processing method and mass spectrometer.
According to various known methods, multiple spectra may be acquired for different time of flight durations i.e. different numbers of passes. The multiple spectra may be compared and together with the knowledge of the mass to charge ratio and the flight time characteristics of the analyser, each detected ion can be assigned a mass to charge ratio value and a mass spectrum can thus be constructed. An example of a method of assigning mass to charge ratio to a multi-pulse Time of Flight spectrometer is described in U.S. Pat. No. 8,410,430 (Micromass).
In fast-pulsing or multiplexed ion mobility separators (“IMS”) wherein multiple packets of ions are introduced into an ion mobility spectrometer or separator device during an ion mobility separation run, and in cyclic or closed-loop multi-pass ion mobility spectrometer or separator systems, wherein ions make multiple passes before exiting the device it is possible for ions of higher mobility to overtake ions of lower mobility during the separation period. As a result, a similar problem exists as with multi-pass Time of Flight mass spectrometers.
GB-2499587 (Makarov) discloses coupling a multipass ion mobility spectrometer with a mass analyser.
US 2006/024720 (McLean) discloses a method of determining the ion mobility of ions in an IM-MS spectrometer, in which ions in an ion mobility separator are multiplexed by plural injection pulses. Ion mobility is demodulated using an ion mobility-mass to charge ratio correlation function.
The above described problem is particularly acute with multi-pass ion mobility separators and is further compounded by the fact that ion mobility spectrometers or separators have a relatively low resolution compared to, for example, Time of Flight mass analysers. This increases the likelihood of peaks representing different ion mobilities overlapping.
Moreover, without information regarding the charge state of the ions it is difficult to assign collision cross section (“CCS”) values to different ions based only on their measured ion mobility.
It is therefore desired to provide an improved method of mass spectrometry.